The disproportionation of toluene involves a well known transalkylation reaction in which toluene is converted to benzene and xylene, often referred to as a Toluene Disproportionation Process or TDP, in accordance with the following reaction:Toluene Disproportionation: Toluene←→Benzene+Xylene  (1)
Mordenite is one of a number of molecular sieve catalysts useful in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite exhibiting a network of silicon and aluminum atoms interlinked by oxygen atoms within the crystalline structure. For a general description of mordenite catalysts, reference is made to Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, 1981, under the heading “Molecular Sieves”, Vol. 15, pages 638-643, which is incorporated by reference herein. Mordenite, as found in nature or as synthesized to replicate the naturally occurring zeolite, typically exhibits a relatively low silica-to-alumina mole ratio of about 10 or less. Also known, however, are mordenite catalysts exhibiting substantially lower alumina content. These alumina deficient mordenite catalysts exhibit silica-to-alumina ratios greater than 10, ranging up to about 100, and may be prepared by direct synthesis as disclosed, for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al, both of which are incorporated by reference herein. Both the typical and the aluminum deficient mordenites are known to be useful in the disproportionation of toluene.
Disproportionation of toluene feedstock may be performed at temperatures ranging from 200° C. to 600° C. or above and at pressures ranging from atmospheric to 100 atmospheres or above and at liquid hourly space velocities (LHSV) of around 1 to 10 hr−1. The specific catalyst, however, may impose constraints on reaction temperatures in terms of catalyst activity and aging characteristics. In general relatively high temperatures are used when employing the high aluminum mordenites (low silica-to-alumina ratios) and somewhat lower temperatures when employing the low alumina mordenites. Accordingly, where mordenite catalysts exhibiting high silica/alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range.
Hydrogen is generally supplied along with toluene to the reaction zone. While the disproportionation reaction (1) does not involve chemical consumption of hydrogen, the use of a hydrogen co-feed is generally considered to prolong the useful life of the catalyst. The amount of hydrogen supplied, which normally is measured in terms of the hydrogen/toluene mole ratio, is generally shown in the prior art to increase as temperature increases. The hydrogen:toluene mole ratio can generally range from 0.05:1 to 5:1.
Nickel-Mordenite TDP catalysts can have good activity and selectivity, but at times produce more non-aromatic compounds than desired. In view of the above, it would be desirable to have a process of conducting toluene disproportionation with a mordenite catalyst with lower production of non-aromatic compounds.